Secretary of State John Kerry testifies before the Senate Appropriations Committee on March 13 in Washington before departing for London. / Win McNamee, Getty Images

by Kim Hjelmgaard and Doug Stanglin, USA TODAY

by Kim Hjelmgaard and Doug Stanglin, USA TODAY

LONDON - Six hours of intense talks Friday between U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov failed to break a deadlock over the Ukraine crisis, with Russian insisting it will back the outcome of Crimea's weekend referendum on secession and Washington calling the election illegal.

The two men met at the U.S. ambassador's residence, breaking away at one point for long, one-on-one talks in a stroll of its manicured grounds in central London.

Both met separately with reporters after the session,with both indication the talks had been "useful" but fruitless.

While the two diplomats both said they would stay in touch, particularly on such issues as Syria and Iran, it was likely the last face-to-face meeting before Sunday's referendum.

Kerry warned of "very serious" consequences if Russia uses the referendum as a "back-door annexation" of the territory on the Black Sea. Those measures could include almost immediate economic sanctions against Russia, including travel restrictions and asset seizures of private Russian businessmen and officials close to Russian President Vladimir Putin.

Lavrov told reporters Friday that such moves would be "counter-productive."

The foreign minister said Russia will "respect" the outcome of the referendum. "Once we have the results, we will announce what we think about it," Lavrov said.

Kerry held out the prospects for a deal that would give Crimea more autonomy and guarantees of protection for Russian minorities, but stop well short of formal annexation by Russia.

The secretary said the West would hold off making any substantial moves until after Putin announces his intentions, but called the decision one of "enormous consequences with respect to the global community."

"If the wrong choices are made, then there will be no choice than to response appropriately because of the gravity of the breach of international standards, the breach of international law, and the challenge, frankly, to the standards by which nations have been called on to behave," he said.

In Washington earlier, President Obama said, "We continue to hope there is a diplomatic solution to be found, but the United States and Europe stand united not only over its message about Ukrainian sovereignty, but that there will be consequences if that sovereignty continues to be violated."

The foreign minister, in his remarks, made it clear there was little real progress on the main dispute -- Russia's military intervention in the Crimea and its heavyhanded moves toward annexation.

In addition, Lavrov said Russia would not participate in any international conference on Ukraine, particularly if it meant holding talks with the current Ukrainian regime, which he described as "illegitimate."

As for Crimea's bid for self-determination, Lavrov noted that there is historical precedent for peoples and regions pursuing self-determination.

"Everybody realizes how important Crimea is to Russia, what Crimea means to Russia," he said. "It means more to Russia than ... the Falklands to Britain."

In other developments, pro-Russia and pro-Ukraine demonstrators clashed overnight in the eastern Ukraine city of Donetsk, leading to the stabbing death of one man while more than two dozen people were injured.

The Russian Foreign Ministry issued a statement Friday on its website blaming those events on Ukrainian authorities, who it said have failed to disarm militants and right-wing groups.

In response to questions about Russian reaction to such unrest, Lavrov said Moscow "does not have any plan to invade east or southern Ukraine."

He added, however, that "we strongly believe that the right of Russian peoples, Hungarian peoples, Bulgarian peoples, and Ukrainians themselves should be protected and secured."

He continued to assert that armed groups that have taken over Ukrainian military bases in Crimea are local,Russian-speaking militias, not Russian troops.